The present invention relates to plasma arc torches and, more particularly, to an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises a metallic tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive insert embedded therein which supports the arc. The insert is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the insert is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More specifically, the emissive insert erodes during operation of the torch, such that a cavity or hole is defined between the emissive insert and the metallic holder. When the cavity becomes large enough, the arc xe2x80x9cjumpsxe2x80x9d or transfers from the emissive insert to the holder, which typically destroys the electrode. To prevent or at least impede the arc from jumping to the metallic holder, some electrodes include a relatively non-emissive separator that is disposed between the emissive insert and the metallic holder. Separators are disclosed in U.S. Pat. No. 5,023,425, which is assigned to the assignee of the present invention and incorporated herein by reference.
U.S. Pat. No. 3,198,932 discloses an electrode for use in a plasma arc torch that attempts to improve the longevity of the electrode and thus the performance of the torch. In this regard, the ""932 patent discloses electrodes having emissive inserts formed from powdered materials, such as zirconium, lanthanum, thorium, or strontium. In addition, silver powder can be added to the powdered materials, which improves the heat transfer from the emissive insert without substantially increasing the work function. The emissive insert is inserted into the holder, which is typically formed of copper, but can also be formed from silver.
Another method used in forming conventional torches as mentioned by the ""932 patent provides securing the emissive insert in the holder by way of brazing. According to this method, the temperature of the brazing material, which is typically silver alloy, is raised to its melting point in order to braze the emissive insert to the copper holder. However, brazing requires an additional manufacturing step and involves the addition of expensive material to the finished electrode.
Thus, it is desirable to retain the benefits of using powdered materials to form the emissive element of a plasma arc torch electrode. It is also desirable to further improve the thermal conductivity of the electrode. It is also desirable to improve thermal conductivity of the emissive element without using a brazing process. Yet it is also desirable to maintain a strong bond between the emissive element and the holder.
The present invention was developed to improve upon conventional electrodes and methods of making electrodes, and more particularly electrodes and methods of making electrodes disclosed in the above-referenced ""932 patent. It has been discovered that the difficulties of the electrodes described above, namely improving the thermal and electrical conductivity of electrodes having powdered metal emissive elements, can be overcome by providing an electrode having thermal conductive paths extending from within the emissive element to a separator positioned between the emissive element and a metallic holder.
This is accomplished by providing an emissive element comprising powders of at least two materials, and a separator that is formed of a material that, according to one embodiment, is substantially similar to one of the materials forming the emissive element. This assembly is inserted in a metallic holder, such as a copper holder, and heated to a temperature such that thermal conductive paths are formed within the emissive element and extend to the separator. After the heating process, the materials of the emissive element have distinct phases, and at least part of the phase of the second material is arranged within the emissive element to form thermal and electrical conductive paths from within the emissive element to the separator. Advantageously, the thermal conductive paths are formed of the material common to both the emissive element and the separator, although the thermal conductive paths can be formed from two or more materials. In one embodiment, the emissive element comprises powders of silver and hafnium, the separator comprises silver, and the thermal conductive paths are formed of silver. It is also possible to add dopants, such as lanthanum oxide, in order to further improve the emissivity of the electrode. The thermal conductive paths improve the performance of the electrode by conducting heat generated by the arc from the emissive element to the separator at a rate greater than electrodes not having thermal conductive paths.
Methods of forming an electrode according to the present invention are also provided. In a presently preferred embodiment, powders from at least two different materials are mixed together, at least one of the materials being emissive. The mixture is deposited within an opening in a separator formed from a relatively non-emissive, electrically and thermally conductive material, such as silver. More specifically, the deposited mixture is compressed into the opening defined by the separator to not less than 60% theoretical (100% theoretical being defined as a solid material having no voids present therein), and preferably to about 80%-90% theoretical.
The combination is heated to define a unitary emissive element bonded to the separator. In particular, the mixture is heated to cause a type of diffusion bonding to take place between the emissive element and the separator. The diffusion bonding results in the formation of the thermal conductive paths between the emissive element and the separator. For example, where the first powdered material comprises hafnium and the second material comprises silver, it is sufficient to heat the mixture to approximately 1400xc2x0 F. to achieve the diffusion bonding and form the thermal conductive paths.
Thus, the present invention provides an electrode and method of making an electrode having improved heat transfer properties over conventional plasma arc torches. By heating powdered materials to form thermal conductive paths between the emissive element and the separator, the emissive element and separator form a relatively strong bond therebetween while improving the thermal conductivity between the emissive element and the separator. In addition, by using a separator being formed of a material substantially similar to one of the powdered materials present in the emissive element, the cost of the electrode is reduced compared to providing an entire metallic holder formed from the same material.